(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for improved hole creation by packing and unpacking of hole patterns and by using alternating phase-shifting masks.
(2) Description of the Prior Art
The creation of semiconductor devices, typically created in or on the surface of a silicon substrate, essentially comprises the creation of numerous interacting patterns and the creation of interconnects that electrically interconnect selected patterns for the creation of operational devices. Device interconnects comprise patterns of interconnect lines, required contact holes or vias are created to connect overlying interconnect lines.
Since device features are transposed from a mask into a semiconductor surface using photolithographic imaging processes, which depends on the transfer of photo energy from a source to a target surface, it is to be expected that, for target features that are created in very close proximity to each other, the transfer of photo energy interacts for very these closely spaced device features, most commonly interconnect lines or contact holes having sub-micron spacing between adjacent features.
A common measure for the occurrence of such interaction are applications where the critical dimensions of the layout of the Integrated Circuit (IC) approach the resolution limit of the lithography equipment. For such applications, proximity effects begin to influence the manner in which mask images are transferred to target surfaces. This interaction imposes limitations on the proximity of adjacent device features, these limitations are referred to as Critical Dimensions (CD) of a design and device layout. This CD is commonly defined as the smallest spacing or the smallest line width of an interconnect line that can be achieved between adjacent interconnect lines. This CD in current technology is approaching the 0.1 to 0.2 xcexcm range.
U.S. Pat. No. 5,573,634 (Ham) shows a contact hole process using a double exposure photoresist process.
U.S. Pat. No. 5,308,741 (Kemp) shows a double exposure process with mask shifting and phase shifting.
U.S. Pat. No. 5,432,044 (Shimizu) shows a double exposure process with a phase shift masks (PSM).
U.S. Pat. No. 6,238,850 (Bula et al.), U.S. Pat. No. 6,162,736 (Nakao), and U.S. Pat. No. 6,134,008 (Nakao) reveal related processes.
A principle objective of the invention is to provide a method of creating contact holes using a packing and unpacking procedure, further applying a phase-shift mask.
Another objective of the invention is to provide a method of creating contact holes of sub-micron dimensions whereby the provided method is not limited to holes of one hole-diameter to hole-distance ratio.
Another objective of the invention is to provide a method of creating contact holes of sub-micron dimensions whereby the range of the ratio of hole-diameter to hole-distance is narrowed, allowing for the creation of more densely packed holes.
Another objective of the invention is to provide improved Depth Of Focus (DOF) during the creation of contact holes.
Yet another objective of the invention is to improve a Mask Error Factor (MEF) associated with the creation of contact holes.
In accordance with the objectives of the invention a new method is provided for the creation of contact holes. The DOF and MEF of closely packed holes can be improved using Alternating Phase Shifting Mask (Alt PSM) for the exposure of the holes. However, Alt PSM are dependent on hole density or hole separation and are less effective where holes are relatively further separated from each other. In order to improve DOF and MEF performance for the creation of holes, the invention adds extra holes to a given pattern of contact holes on the surface of a first mask, thus densifying the pattern of holes on the first mask and therefore reducing the range of the hole-diameter to hole separation ratio. The pattern of added holes is alternating in phase with the pattern of desired holes. The added holes will be filled up using a second mask.